a) Field of the Invention
The invention is directed to a method and an arrangement for cleaning optical surfaces of reflection optics which are contaminated in a lithographic exposure device by debris emitted by a hot plasma of a plasma-based EUV radiation source, particularly of collector optics in extreme ultraviolet (EUV) high-power radiation sources for semiconductor lithography.
b) Description of the Related Art
EUV radiation is usually generated by thermal radiation sources, particularly by the generation of dense, hot plasmas which are typically based on laser-produced plasma (LPP) or gas-discharge plasma (GDP) and emit isotropically in space. Therefore, for application of EUV radiation, collecting optics are arranged near the source to bundle the emitted radiation from the largest possible solid angle.
A characteristic feature of any plasma-based radiation source is that, in addition to the desired short-wavelength radiation, fast ions or neutral particles are also emitted from the plasma in all directions. These particles (debris) are damaging primarily to the collector optics and condenser optics near the plasma which are constructed for the EUV range as reflection optics with sensitive surfaces, whether they are normal-incidence multilayer mirrors or grazing-incidence metal mirrors. In either case, the surfaces are degraded by the impinging debris, above all in that debris particles are deposited on the surface and reduce reflectivity.
Aside from xenon, it has become increasingly common to use tin or lithium as a working medium in high-power radiation sources for EUV semiconductor lithography because they achieve a higher efficiency of energy conversion in the wavelength range around 13.5 nm. Tin vapor, lithium vapor or eroded electrode materials (e.g., tungsten or molybdenum) which are generated at the source or in its immediate surroundings condense on the relatively cool surfaces of the optics and are deposited there as layers. Deposits of this kind alter the surface characteristics and reduce the reflectivity of the optics relatively quickly.
Cleaning methods using reactive gases or gas radicals are known in the prior art for removing deposits on EUV optics. For example, US 2004/011381 A1 describes the cleaning of optics contaminated by carbons and hydrocarbons by means of atomic hydrogen especially for multilayer optics. The free hydrogens are generated within a closed, cooled housing by flowing through hot filaments and are directed to the optics as a flow of gas. The description refers to an in-situ cleaning, but this can scarcely be implemented in EUV sources for semiconductor lithography simply for space considerations, as the space in front of (normal-incidence) multilayer optics is reserved for generating the EUV-emitting plasma.
Further, for cleaning light source collector optics for grazing incidence, U.S. Pat. No. 6,968,850 B2 discloses the elimination of a tungsten coating of eroded electrode material through free fluorine in that the collector optics in the shape of an ellipse of rotation are divided into sections (e.g., half-shells) to which different potentials are applied in order to generate a plasma from the fluorine gas introduced therebetween by microwave excitation or HF excitation, the plasma forming gaseous compounds with the eroded electrode material (tungsten) which can then simply be pumped off.
Glowing filaments are often used to regenerate reactive gases for thermal splitting of the molecules of a gas (e.g., hydrogen). For example, US 2004/011381 A1 describes a method for cleaning optics with hydrogen especially for Mo/Si multilayer optics in which atomic hydrogen is directed to the optical surface. The device indicated for this purpose for generating the flow of hydrogen resembles a cooled blow dryer in which molecular hydrogen is introduced on the input side and activated by a heated filament grid so as to be expelled on the output side. However, the hot filaments are disadvantageous in that they must likewise be replaced at regular intervals and even cause contamination through evaporation. Further, the spatial distribution of the reagents is inhomogeneous in small filaments and there is scant possibility of integrating large-surface (close-meshed) filaments in the optical systems of an EUV source without substantial radiation shadows because collector optics must usually occupy an entire half-space around a virtually punctiform plasma to achieve the highest possible efficiency in beam bundling.
Further, US 2006/0000489 A1 discloses microwave plasmas or HF plasmas for generating free fluorines or fluorocarbons, wherein free hydrogen or free oxygen must then be generated to eliminate the reaction products of the fluorine-containing radicals. The free hydrogens or free oxygens are likewise formed either through microwave excitation or HF excitation or also through the interaction of corresponding molecules with fluoro-plasma. There remains the difficulty of generating the radicals with sufficient uniformity that they reach every location of a nested optical system.
An arrangement for improving the homogeneity of a flowing plasma and the efficiency of the coupling-in of energy was described by D. Korzec, et al. in “Characterization of a slot antenna microwave plasma source for hydrogen plasma cleaning”, J. Vac. Sci. Technol. A 13, 4 (1995) 2074-2085. For this purpose, a hydrogen plasma is generated in that microwave energy is coupled from the periphery into an internal, cylindrical quartz tube through which gas flows longitudinally by an annular waveguide through an inwardly directed slot antenna system (SLAN). To this end, a standing electromagnetic wave is generated in the annular waveguide and the antenna slots are arranged in the inner wall of the annular waveguide at its wave nodes in order to achieve a maximum, evenly distributed coupling in of the microwave energy. The drawback in this case is the spatially determined, compact construction that prevents integration into a plasma-based EUV source.
Further, it is known from WO 2005/101122 A1 for the removal of carbon-containing deposits that are deposited on a multilayer mirror during the generation of EUV radiation to generate volatile carbon compounds through chemical reactions with nitrogen or halogens which can then be removed by suction. This solution, which is described only for discharge plasmas of O2 and H2, has the drawback that the latter are only usable ex situ for optical systems which are contaminated in EUV radiation sources.
US 2007/0062557 A1 discloses an electric discharge generator for cleaning an optical element of a lithographic exposure device. In one variant, the discharge generator is integrated directly in the collector mirror by arranging a plurality of current-carrying coils around the outer sides of a plurality of nested, rotationally symmetric reflectors of the collector mirror in order to generate by induction a high-frequency discharge in the gas between the individual mirror shells. In a modified construction, the rotationally symmetric reflectors are provided with conducting plates on their outer sides. Oppositely located conducting plates of neighboring reflectors are insulated from one another and high voltage is applied to them by the discharge generator in order to generate a great number of capacitive discharges between a plurality of oppositely located conducting plates of neighboring reflector shells. The disadvantage in this solution is that fast electrons as well as fast ions are generated in the gas that flows through and, as a result of the electric and/or magnetic field effect, are accelerated to the nested metal reflector shells so that the generated plasma causes thermal stress on the reflector surfaces and additional unwanted sputtering effects result from the accelerated ions.